Loran-C is a well-known, long-range, high-power, precision navigation system employing automatic envelope detection and radio frequency cycle comparison techniques. A Loran-C chain consists of one master and at least two secondary transmitters, with each individual chain being identified by a specific group repetition interval (GRI). The signals which are radiated by the transmitters are pulsed electrical signals that are very accurately timed, and phase coded for identification. These signals are propagated both as groundwaves and as skywaves reflected from the ionosphere. The standard measurement technique consists of measuring the times of arrival of signals from two secondary transmitters, A and B, relative to the time of arrival of the signal from a third transmitter M (master). These two time differences TDA and TDB identify two hyperbolas that intersect at the receiving antenna. The precision of position location available from Loran service may then be seen to be dependent upon the precision of the time difference measurements. However, while the need for measurement accuracy may seem obvious a further requirement is speed of measurement. When Loran is being used for marine navigation there is a relatively large amount of time in which to acquire the signals and perform the measurements, but when Loran is being used for aircraft navigation the high speed makes rapid signal acquisition and speedy measurement taking a necessity.
When used on high-speed aircraft it is probable that the flight will pass through any number of Loran triads and chains. Triads are referred to because three transmitters are required to produce the two hyperbolic lines of navigation. A further system limitation currently exists because, since present Loran receivers operate on only one group repetition interval at one time, all three transmitters must be members of the same Loran chain. Also, most present receivers require the master transmitter to be a member of each triad.